Plasma reactors are used for a number of steps in the fabrication of semiconductor integrated circuits including plasma enhance chemical vapor deposition (PECVD), magnetron sputtering or more generally physical vapor deposition (PVD), etching, and cleaning as well as other general plasma processing for conditioning or treating one or more layers of the circuits being developed on a wafer. Modern plasma processing equipment tends to be very expensive because it needs to perform within a wide narrow processing window and be capable of operating with little down time for repair and maintenance. Thus, it is desired to widen the processing window and reduce down time without significant increasing the cost.
One problem in the operation of plasma reactors is that material tends to deposit on the parts of the chamber other than the wafer being processed. Deposition can occur not only in deposition chambers such as used for PECVD and PVD, but also in plasma etch and cleaning reactors in which the etching process removes material from the wafer but deposits etchant residues on the chamber walls and other chamber parts. An example of a plasma etch reactor is the DPS etch reactor 10 available from Applied Materials, Inc. of Santa Clara, California and schematically illustrated in the cross-sectional view of FIG. 1. It includes a chamber body 12 and chamber lid 14 defining a processing chamber 16 and pumping chamber 18 enclosing a cathode 20 on top of which an electrostatic chuck 22 or other wafer holding device supports a wafer 24 to be etched. An edge ring 25 surrounds the wafer 24 during processing.
The wafer 24 is inserted into the processing chamber 16 through a wafer port 26 selectively closed by a slit valve door 28, also illustrated in the orthographic view of FIG. 2, to allow a vacuum pump 30 to pump the pumping chamber 18 and processing chamber 16 to a pressure typically in the mid-milliTorr range. Processing gas is metered into a gas line 32 from a gas supply 33 and injected into the processing chamber through a gas nozzle 34 on the bottom of the lid 14. A heater 36 selectively heats the lid 14 and hence the wafer 24. An RF coil 38 on the back of the lid 14 inductively couples RF power into the processing chamber 16 to excite the processing gas into a plasma to etch the wafer 24. The cathode 20 may be RF biased through a capacitive coupling circuit to assist in the etching process especially in the formation of high aspect-ratio holes. A screen 40, also illustrated in the orthographic view of FIG. 3, separates the processing and pumping chambers 16, 18 while allowing gas flow between them. It is electrically grounded to confine the plasma to the processing chamber 16.
The etching process tends to produce etch residues which deposits around the sidewalls of the processing and pumping chambers 16, 18, which require periodic cleaning. Furthermore, it is desired that the chamber body 12 be made of an inexpensive metal like aluminum while more etch-resistant and expensive materials line the processing and pumping chambers 16, 18. Accordingly, a lower liner 42, also illustrated in the orthographic view of FIG. 4, lines the pumping chamber 18 and an upper liner 44, also illustrated in the orthographic view of FIG. 5, lines the sides of processing chamber 16 and also defines part of the top of the pumping chamber 18. Similarly, a cathode liner 46, also illustrated in the orthographic view of FIG. 6, lines the lateral sides of the cathode 46.
After extended usage, which likely produces a surface coating of fluoride residue, the liners 42, 44, 46 as well as the slit valve door 28 and screen 40 may be removed from the reactor 10 to be replaced by fresh parts. Advantageously, used parts are cleaned and returned to service.
Liners and other removable parts of the chamber are often composed of aluminum. Particularly, in a metal etch reactor, aluminum itself is to some degree subject to the same etching chemistry to which the metal parts of the wafer are exposed. Even in silicon and dielectric etch chambers, the chemistry for etching the silicon or dielectric layer tends to also etch aluminum chamber parts. Accordingly, it is conventional in many plasma etch reactors to anodize aluminum chambers bodies and chamber parts to create a surface layer of anodized aluminum, generally of the composition Al2O3, which is often substantially non-reactive with metal etching plasmas. Type-III hard anodization is typically performed using a hot H2SO4 electrolyte to produce a thick, hard, black surface layer. As illustrated in the cross-sectional view of FIG. 7, anodization oxidizes an underlying aluminum part 60 acting as a substrate. It is understood that aluminum may contain up to about 10 wt % of unintentional impurities or intended alloying components such as silicon. The anodization initially creates a thin fairly uniform and dense interfacial layer 62 of anodized aluminum. However, further anodization produces a thicker columnar region 64 of anodized aluminum in which columns 66 form around pores 68. The electrolyte can fill the pores 68 to efficiently grow thick anodized layers. The pores 68 allow the columns 66 to grow upwardly relatively far away from the aluminum source material.
The solid portions 62, 64 of the anodized aluminum provide a relatively robust protective layer for the underlying aluminum part 60. However, the pores 68 provide a passageway for the etching plasma to reach close to the aluminum part 60. Accordingly, it is standard practice to seal the pores 68 with a sealing layer 70 illustrated in the cross-sectional view of FIG. 8, which prevents the entry of the etching plasma into the pores 68. A conventional sealing layer 70 is composed of boehmite, which is (—AlO(OH), that is aluminum hydroxide. The boehmite sealing layer 70 may be formed by immersing the anodized surface in hot deionized water (DI).
A fluorine-based plasma etch presents particular problems for anodized aluminum chamber parts. A typical process gas for etching metal includes a fluorocarbon CxFy although other gases may be added and other fluorine-based etching gases include NF3, SF6, and other fluorides. Dielectric etching processes similarly use plasmas of fluorocarbon gases. The fluorine in the presence of aluminum parts, even anodized parts, is likely to produce aluminum fluoride (AlFx) and aluminum oxyfluoride (AlOF), which are non-volatile and deposit on the anodized part and begins to thicken. Although particles are a potential problem, a more immediate problem is that the thickening aluminum fluoride or oxyfluoride film tends to change the chemistry of the chamber and causes the etching process to drift. Unlike most other fluorides or halides, aluminum fluoride is a non-volatile and is a hard material insoluble in water and many solvents. No plasma cleaning process has been found that can remove the aluminum fluoride during an in situ plasma dry clean. A further problem is that the aluminum fluoride tends to be covered by a thin layer of glass silica (SiO2), formed either from quartz chamber parts or as reaction by-products from etching a silicon or silicon oxide layer.
Conventional cleaning processes have used heavy abrasive scrubbing to remove the AlF and silica layer. However, the heavy scrubbing tends to damage the anodic coating. Sometimes dilute oxalic acid (C2H2O4) is used as a wet cleaning solution. However, oxalic acid does not effectively remove the by-products and residue from the wafer processing. Further, the acid solution tends to degrade the anodization and the sealing layer.
It is possible to simply replace chamber parts with fresh and newly anodized parts. However, parts tend to be expensive and the aluminum vacuum chamber itself needs occasional cleaning.